1. Field of the Invention
The present invention relates to a private branch exchange to which multifunctional telephone terminals can be connected, and particularly relates to a method of transmitting control signals for a high speed transmission of a lot of information to and from such terminals.
2. Description of the Prior Art
A private branch exchange (PBX) in recent years serves many analog or digital multifunctional telephones as well as the so-called standard telephone terminals of analog type, providing a variety of functions. Some of the multifunctional terminals are provided with their own displays to greatly enhance the operational performance by displaying the phone number and/or name of the other party at the time of originating a call or during a call, and/or by displaying information such as operation modes of them.
Further, with recent evolution in technology of integrating a computer and a PBX, displays provided for multifunctional terminals is becoming large, so that they, having access to the computer and its terminal independently of intrinsic functions of a telephone terminal, retrieves various kinds of information or offers reservation services. Prevailing among interfaces between a PBX and multifunctional terminals, is those of a type providing one controlling and two speech channels in a digital transmission system, where generally speech channels are of 64 Kbps in transfer rate and controlling channels are of 16 Kbps.
The above described controlling channel may be said to have a channel capacity or a transfer rate enough to display key information of a multifunctional terminal and control lighting amps, and further to control display of a relatively small display device. However, if multifunctional terminals with a large display being connected, a large amount of control information is to be displayed in a moment on the large display as is practiced nowadays, it is hard to say that the transfer rate of the current controlling channel is satisfactory. In order to send a large amount of control information in a moment to the large display, the transfer rate of the current controlling channel might be raised, but it would lead to an increase in the frequency of the clock, which would accordingly necessitate an enhancement in the precision of circuits relating to information transmission in response to the increase in the clock frequency.
This approach is disadvantageous not only in that it makes circuits complicated causing the cost to become high, but also in that even if a controlling channel with a high transfer rate is used, the transfer rate can not be made effective use of, which is not economical, because most of the control signals transmitted through the controlling channel are supported by terminals having small displays and according have a low information density. Thus, there is a demerit in a prior art system that if a large amount of information is to be transmitted, for display, in a moment to a multifunctional terminal having a large display, there is no available means for transmitting a large amount of information at a high rate between the terminal and the PBX system. Though even a current controlling channel permits a transmission of a large amount of control information to a large display only if taking time is permitted, a prompt access to a large display is impossible causing operational performance to deteriorate; this becomes a problem.
Since as described above, there is no means for transmitting a large amount of control information at a high rate between the terminal and the PBX system, if a large display is adopted as a display of a telephone terminal, it takes too much time to display the whole information on a large display as long as a current controlling channel is used for transmission of display information etc., which, disadvantageously, prevents the use of a large display which enables a large amount of information to be displayed, significantly imposing restrictions on the functions of telephone terminals.